Shock Processes in Porous Quartzite: Transmission Electron Microscope Observations and Theory

A high-resolution study of shocked Coconino Sandstone from Meteor Crater, Arizona, was undertaken using transmission electron microscopy to investigate the textural relations of high-pressure phases produced by meteorite impact. In weakly shocked rocks (estimated average pressure, P < 100 kb), quartz in the interiors of grains retains its initial microstructure, but near original grain boundaries, quartz is altered by fractures and planar features resembling Brazil twins, and is partially transformed to coesite and glass. In moderately shocked rocks (estimated average pressure, 100 <P<250 kb), as much as 50% of the residual quartz is fractured but otherwise undeformed. Near grain boundaries relatively undamaged quartz exists in direct contact with coesite and stishovite. FilamentarY, microvesicular '~ f roth" fills cracks and fractures in the regions containing high-pressure phases. Coesite present in regions which are collapsed pores has a unique texture, not previously reported for a shock-formed phase: the grains are equidimensional and form a mosaic pattern characteristic of products of high-temperature recrystallization. In strongly shocked rocks (estimated pressure P < 250 kb) quartz contains abundant glass lamellae, identical to optical "planar features" except that they are so closely spaced that they would not be resolved optically. Vesicular glassy regions in strongly shocked rocks contain remnants of large ( ~5 gm) coesite crystals, indicating that the shockformed glass in these regions formed by melting of coesite rather than quartz. The textural relationships of coesite, stishovite and glass observed in these rocks provide evidence regarding the processes of the formation and destruction of high pressure phases during the passage of a shock wave. Three types of coesite are observed: (1) Polycrystalline coesite which formed directly from quartz grains, perhaps with topotactic control; (2) Single-crystal coesite grains which have partially inverted to form thetomorphic coesite glass; (3) Well-equilibrated coesite which nucleated and grew from a hot precursor phase, believed to be amorphous silica with silicon in six-fold * Permanent address: Department of Physics, Monash University, Clayton, Victoria, Australia 42 s.w. Kieffer et al. coordination. The texture of the stishovite, found only in the moderately shocked rocks, leads us to conclude that it formed by direct nucleation and growth from quartz grains. We believe that only a small amount of stishovite was formed and that the stishovite which was formed did not invert to glass. Three types of glass are observed: (1) Thetomorphic coesite and quartz glass, formed by the inversion of the crystalline phases; (2) Glass (lechatelierite) pervaded with relatively large spheroidal vesicles and schlieren, generally formed by melting of coesite crystals; (3) Glass ("froth") pervaded with vesicles of irregular, generally nonspheroidal shape, having diameters of tens of Angstroms, formed by the interaction of quartz, coesite or glass with hot water vapor. A detailed description of the reaction of porous sandstone to the passage of shock waves of milliseconds duration is derived and shock Hugoniot data for single crystal quartz, water, and wet and dry rocks are reviewed in order to provide pressure estimates for each type of rock and to provide a basis for comparing naturally shocked samples with laboratory data. In weakly shocked rocks, pore closure is accomplished by brittle fracture of grain s and rotation of fragments into pores. In moderately and strongly shocked rocks, pore closure is accomplished by jetting, the extrusion of molten streams of hot material into the pores, forming cores of extremely hot material from which the well-equilibrated coesite aggregates crystallized. In the moderately shocked rocks, these coesite crystals are preserved within the cores. In strongly shocked rocks, most of the coesite melts to form lechatelierite. The history of water contained within pores during passage of a shock wave is complex. In moderately shocked rocks, hot steam forms and reacts with quartz, coesite and stishovite in the vicinity of the collapsed pores. The hot steam erodes grains in some places, forming a silica-rich fluid which can subsequently be deposited in other places in the rocks. In strongly shocked rocks, silica and hot water intermix to form a super-critical SiO2H20 solution, from which the water exsolves late in the rarefaction event to form vesicular glass.